a. Field of the Invention
This invention pertains generally to devices and methods for obtaining and processing differential electrical signals. More particularly, this invention pertains to devices and methods for obtaining and processing differential electrical signals in medical monitoring and diagnostic systems. However, it should be understood that this invention may be used in a broad range of applications where differential electrical signals are measured and recorded or displayed. Thus, although this invention is described in terms of its use in medical monitoring and diagnostic systems, it should be understood that the scope of this invention extends in general to any system in which differential electrical signals are measured and recorded or displayed.
b. Description of the Prior Art
A common technique used for medical diagnosis is the measurement and display as a function of time of the differences between the electrical potentials (voltages) measured at two or more different points on the human body. The electrocardiograph ("ECG") and the electroencephalogram ("EEG") are well known and widely used examples of this technique.
Many sources of electrical potential contribute to the potential that is measured at one point on the human body. Muscular activity and other organic activity throughout the body contribute to the electrical potential, as well as sources such as electrical power systems and other electrical equipment that are external to the human body. In order to remove or cancel out the effects of external sources and the effects of internal sources that are remote from the area in the body that is being examined, one measures the difference in potential (i.e. the potential differential) between two points on the human body which are located in the area of the body being examined. For instance, in a typical ECG examination, a reference wire is attached to the patient and a series of wires are attached to various places on the patient. Differential voltages which represent the differences in potential between various pairs of the wires connected to the chest are measured and displayed. The voltages (or potentials) are sometimes referred to more generally in this specification and in the claims as signals.
Referring to FIG. 1, in the simplest configuration wires 10 and 11 are connected to two electrical potentials (voltages) for which a differential is to be measured. Wires 10 and 11 are connected to a differential amplifier 12 and the output is then recorded and/or displayed as a function of time. As shown in FIG. 2, in the prior art devices buffer amplifiers 21 and signal conditioners 22 typically are connected between each input wire and the associated differential amplifier 23. The buffers provide high impedance inputs which isolate the potentials at the measurement points on the body or other source from the measuring equipment. The signal conditioners amplify the sensed potentials, limit the bandwidths of the signals that are passed to the differential amplifier and may also provide adjustable or fixed offsets in the voltages that are output to the differential amplifier.
In most modern systems, an analog to digital converter 24 converts the analog signal, that is output from the differential amplifier, into a digital signal for display on, and/or storage in, appropriate digital devices. The analog to digital converter 24 outputs a sequence of digital numbers, each number representing the value of the input voltage at an instant in time and the sequence of numbers representing the values of the input voltage at successive instants in time.
In addition to measuring potentials at the body surface, probes have been developed for insertion into the human heart to measure the electrical potentials as a function of time at different points within the heart. A single probe may contain as many as sixty (60) or more separate wires with each wire connected to a separate electrical contact on the surface of the probe. It is likely that the number of contacts on the surface of such probes will be increased substantially in the near future. By measuring the differences in the electrical potentials (the differential voltages) appearing on the wires, the diagnostician is able to obtain a "map" of the electrical activity of that portion of the heart that is in contact with the surface of the probe.
The prior art equipment for measuring the differential voltages suffers from the limitation that the particular differential voltages, that are to be measured and displayed and/or recorded, must be selected prior to the measurement. A pair of wires corresponding to each selected differential is then connected to a differential amplifier and the test is performed the patient. If, after examining the data, the diagnostician wishes to view the difference in potential between two contacts for which the differential was not previously selected, the diagnostician must then make new connections to the differential amplifiers and repeat the tests on the patient. One could avoid the problem of having to perform the test again by using measurement equipment that included a differential amplifier and an analog to digital converter for each possible combination of contacts, taken two at a time. As should be apparent, however, if the probe contains more than a very few contacts, the number of possible combinations, i.e. the number of differentials to be measured, becomes very large and the cost of the related measuring equipment becomes prohibitive.